1. Field of the Invention
The present invention relates in general to a mat for reducing erosion and, more particularly, to a rigid transition mat secured to hard armor and extending over the transition area from a hard armor erosion control surface to a soft armor erosion control surface.
2. Description of the Prior Art
The Clean Water Act and subsequent legislation requires storm water to be discharged in a non-erosive manner. Unfortunately, storm water pipe outlets and the like used to divert water runoff are highly erosive at their outlets as the result of velocity and shear force problems associated with the funneling of water toward a narrow outlet. Erosion control associated with such outlets involve economic, physical and logistical problems. Traditionally, storm water is transported from a street or parking lot in a storm water pipe to a conveyance, such as a stream or river. Storm water may also be drained from a permanent structure, like a parking lot, at designated outlets where it flows overland and naturally dissipates. The soil area adjacent such discharge points is highly susceptible to severe erosion associated with discharging water.
The energy of water discharging from such outlets varies with the of velocity, shear force and volume of the effluent. Water 25 centimeters deep, flowing rapidly, is much more erosive and destructive than water 8 centimeters deep, flowing at the same rate. Accordingly, allowing runoff water to spread out is an effective means to counteract funneling of discharge water, dissipating both velocity and shear force without mechanical input. Conversely, squeezing water raises its height and increases its hydraulic pressure. This increase in hydraulic pressure results in increased shear force which, in turn, leads to increased erosion. Unfortunately, the factors associated with diverting water, namely collecting water from a relatively large area and funneling it to a very small area, using hard, smooth surfaces, cannot help but magnify the weight, velocity and shear force of the water at the discharge point.
Traditionally, at such discharge points, material, such as rip rap, is added. Such installation of various sized rocks, stacked in a concave manner to funnel water, may be used to reduce erosion, but is very expensive and time consuming to install. Alternatively, concrete blankets (flat soft material filled with concrete or concrete blocks held together with steel cables), or concrete slabs may be used to control erosion at discharge points. These products, and other similar products, are referred to as “hard armor.” Hard armor often dissipates water energy and protects the soil therebeneath from eroding away and polluting natural resources. One drawback associated with hard armor is the requirement of very large equipment needed to install the hard armor. Additionally, a significant volume of material must be freighted to the site and a large amount of preparatory work is required before installing the hard armor.
While hard armor is useful for dissipating velocity and countering shear forces associated with runoff water, poor installation often allows the water to splash or divert out of the designated channel, many times leading to the erosion and washout of the hard armor installation itself. While concrete blankets are better able to withstand velocity and shear forces, they do little to inhibit the velocity and, therefore, the destructive force of water runoff. Another drawback associated with hard armor is that it typically lacks aesthetics associated with other forms of erosion control.
Recently, the industry has developed blanket-type products called turf reinforcement mats to convey water and withstand designated loads. While such turf reinforcement mats do little to reduce or mechanically dissipate the energy of runoff water energy themselves, their installation allows for the growth of vegetation which, in turn, mechanically reduces energy associated with runoff water. Such blankets are typically three-dimensional, flexible mats constructed of plastic webbing. The open weave of such mats allows vegetation to grow up therethrough. The combination of the mechanical stable structure and open weave design results in a significant synergistic effect, with the capacity to carry much greater velocity and sheet force load because roots and stems associated with the upgrowing vegetation are reinforced by the mat.
One drawback associated with such turf reinforcement mats is the inability to gain sufficient vegetation growth before the energy associated with runoff washes the seeds or small plants away. Moreover, if sufficient vegetation does not occur, the mats often fail from soil erosion. The greatest incidents of failure of such turf reinforcement mats, canvas and other associated types of “soft armor,” occur at butt connecting joints, either between two pieces of soft armor, or between soft armor and hard armor. Typically specifications call for trenching perpendicular to the flow of water, and overlapping or wrapping of blanket material in the trench area. The trench is then filled with soil, packed, and the blanket material folded back against the trench.
If, as is often the case, the soil is inadequately compacted at this trench, runoff seeps into the trench, washing away the soil contained therein, and leading to failure of the trench retainment construction. It would, therefore, be desirable to provide an erosion control system which avoided the failure problems associated with soft armor, and which avoided prior art problems associated with soft armor transition areas and transition areas between hard armor and soft armor. The difficulties in the prior art discussed hereinabove are substantially eliminated by the present invention.